1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a coating-type magnetic recording medium incorporating a magnetic coating film as a magnetic layer thereof and a backcoat layer formed on another surface thereof and arranged to be used in a data media.
2. Description of the Related Art
In recent years, magnetic recording mediums have been widely employed for audio apparatuses, video apparatuses, computers and the like. The need for the magnetic recording mediums has considerably been raised.
In general, a magnetic recording medium of the foregoing type has a structure that a magnetic layer composed of magnetic powder and a binder is formed on a non-magnetic support member constituted by a polyester film and the like. Moreover, a backcoat layer composed of non-magnetic powder and a binder is formed on another surface of the non-magnetic support member. Usually, the backcoat layer is formed by applying or transferring, to the non-magnetic support member, a backcoat coating material in which non-magnetic powder is dispersed in a composition containing the binder.
Hitherto, the binder has been made of an organic polymer compound, such as polyester resin, cellulose resin, polyurethane resin, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinylidene chloride copolymer, acrylonitrile-butadiene copolymer, ferrol resin, epoxy resin, polyamide resin, polyvinyl butyral, nitrocellulose, cellulose acetate butyrate, acrylic resin or electron-ray setting resin.
Since the foregoing binders have advantages and disadvantages, sole use of the binder does not permit a magnetic layer or a backcoat layer having required characteristics to easily be obtained. Therefore, two or more types of the binders are combined with one another. For example, combination of relatively hard resin, such as vinyl chloride resin, polyvinyl butyral or nitrocellulose, and soft resin, such as polyester resin, polyurethane resin or acrylonitrile-butadiene copolymer, is frequently employed. To improve the durability of the magnetic layer and the backcoat layer, a polyisocyanate compound is frequently employed as a hardening component.
In recent years the capacity of the magnetic recording medium has considerably been enlarged. As a means for enlarging the capacity, the coating-type magnetic recording medium has variously been devised such that the recording method is improved by reducing the track pitch or by shortening the recording wavelength or the overall length of the medium is elongated. In particular, the method with which the recording wavelength is shortened requires the adaptable medium to have a high recording density and to be capable of producing a large output.
The foregoing high-density recording medium is usually provided with a backcoat layer in order to maintain a required movement characteristic with respect to a drive.
The backcoat layer must have a required movement characteristic with respect to the corresponding drive. To preserve data, the initial characteristics must be maintained even if the recording medium is reserved for a long time or even after the recording medium has been reserved in a hot and wet condition. Since the backcoat layer is in direct contact with the ground magnetic layer during the reservation, excessive roughness of the surface shape of the backcoat layer causes the rough surface shape to be transferred to the magnetic layer. In the foregoing case, dropout increases and/or the error rate is undesirably raised. There is apprehension that sticking between the backcoat layer and the magnetic layer occurs or corrosion takes place according to the selected material which constitutes the backcoat layer. The magnetic layer of the high-density recording medium must inevitably have a mirror surface. Also the structure of the magnetic layer is usually constituted by a coating-type medium containing magnetic alloy powder. Therefore, the backcoat must meet furthermore severe requirements to realize the above-mentioned reservation characteristics.
In general, the backcoat layer has a structure that non-magnetic powder is dispersed in a binder. In particular, a structure is usually employed in which carbon black is, as a main component, dispersed in the binder and other carbon black, a variety of pigments or a variety of additives are blended under certain circumstances. The reason why the foregoing carbon black is employed in the backcoat is to lower the surface electric resistance of the backcoat layer, prevent adhesion of dust owing to static electricity, impart a light shielding characteristic to prevent malfunction and improve the durability against movement.
An example of use of carbon black as the non-magnetic powder has been disclosed in Japanese Patent Publication No. 52-17401. The foregoing disclosure is structured to obtain an antistatic effect using the conductivity of carbon black, a light shielding effect and a surface coarsening effect owing to coagulation of carbon particles. However, carbon black of a type having a particle size of 10 nm to 20 nm cannot easily be formed into a coating material. If carbon black is formed into the coating material, coagulated particles are easily formed. As described above, carbon black having the small particle size and suffering from a defective dispersion characteristic cannot easily be formed into a structure having an average roughness with which the foregoing asperities can be prevented. The reason why dispersion cannot easily be performed is considered that carbon black having an excessively small average particle size easily has secondary structures.
In Japanese Patent Laid-Open No. 63-144416, an example has been disclosed in which thermal black type carbon black having a particle size of 60 nm to 200 nm is employed. Carbon black of the foregoing type having a small number of structures can uniformly be dispersed in the binder. Moreover, the friction factor can effectively be reduced. However, prevention of asperities is limited owing to the particle size of carbon black after the uniform dispersion has been prevented. Therefore, carbon black of the foregoing type cannot be used to form the backcoat of a high-density recording medium.
An example arranged to use a plurality of types of carbon black has been disclosed in Japanese Patent Publication No. 2-49490. A main object of the foregoing disclosure is to improve wear resistance. In the foregoing disclosure, carbon black having a particle size of 10 nm to 35 nm and another type carbon black having a particle size of 40 nm to 150 nm are employed. In Japanese Patent Publication No. 4-81261, small-size carbon black having an average particle size of 30 nm to 100 nm and coarse carbon black having an average particle size of 150 nm to 500 nm are employed. However, simple employment of both of the small particle size and the large particle size of the carbon black cannot easily simultaneously meet required movement characteristics and reservation characteristics of the high-density recording medium.
A multiplicity of attempts have been made to employ both of carbon black and another inorganic pigment as the non-magnetic powder. For example, in Japanese Patent Publication No. 5-72647, use of carbon black and addition of inorganic pigment, such as CaCO.sub.3, BaSO.sub.4 or Fe.sub.2 O.sub.3, having a Moh's hardness of 6 or smaller has been disclosed in order to reduce the quantity of undesirable cutting of the surface of the backcoat. Since the binder is made of vinyl chloride copolymer or fiber-type resin, the foregoing structure serving as the backcoat of a coating-type medium incorporating magnetic alloy powder cannot satisfactory be reserved in a hot and wet environment.
Another attempt has been made in which a plurality of carbon materials and another inorganic pigment are combined with one another. In each of Japanese Patent Laid-Open No. 2-42624, Japanese Patent Laid-Open No. 2-42625, Japanese Patent Laid-Open No. 2-134720 and Japanese Patent Laid-Open No. 2-141925, simultaneous employment of carbon black having an average primary particle size of 20 nm to 40 nm, carbon black having an average primary particle size of 50 nm to 100 nm and another pigment has been disclosed. Each of the foregoing disclosures arranged to use fiber resin as the binder cannot be used as the backcoat of the coating-type medium incorporating magnetic alloy powder because satisfactory preservation in a hot and wet environment cannot be permitted. The foregoing inorganic pigment employed to adequately roughen the surface and having an average particle size of 300 nm to 1500 nm encounters a problem of transfer of the backcoat surface.
The employed binder has an excellent characteristic for permitting dispersion of non-magnetic powder, satisfactory adhesiveness with the non-magnetic support member and sufficient wear resistance of the formed film. For example, a thermosetting material is employed which is obtained by combining one or more types of the following resin materials: polyurethane resin, polyester resin, fiber resin, vinyl chloride copolymer resin and phenoxy resin is solely employed and a polyisocyanate compound. As an alternative to this, combination with resin having a radioactive-ray functional unsaturated double-bond is employed.
However, vinyl chloride copolymer resin or the fiber resin which has possibility of producing corrosive gas is not suitable as the binder for the backcoat of the coating-type medium which incorporates magnetic allow powder. Structures each of which does not incorporate the vinyl chloride copolymer resin or the fiber resin has been disclosed in Japanese Patent Laid-Open No. 58-200426 and Japanese Patent Laid-Open No. 59-2228. The binder is constituted by phenoxy resin, thermoplastic polyurethane elastomer and polyisocyanate. The foregoing binder system is, however, employed to simply and mainly improve the initial durability against movement. That is, improvement in the reservation characteristic is not attempted. Moreover, there is no discussion about the dispersion characteristic of the contained polyurethane elastomer. The polyurethane elastomer described in the embodiment has no polar group. A binder system obtained by combining polyurethane elastomer and phenoxy resin cannot sufficiently disperse inorganic powder. A filler employed in the foregoing disclosures is constituted by inorganic powder of CaCo.sub.3 or the like or carbon black. Each of the foregoing components is employed solely. No description is made about use of a mixture of two or more materials above. Therefore, maintaining of the conductivity, reduction in the surface electric resistance, prevention of adhesion of dust owing of static elasticity and improvement of the wear resistance of the backcoat surface cannot simultaneously be realized. Although a multiplicity of suggestions to impart the polar group to the polyurethane elastomer have been made, any suggestion for improving the dispersion characteristic of carbon black in the mixture system with phenoxy resin has not been made.
In Japanese Patent Publication No. 1-91317, Japanese Patent Laid-Open No. 6-325353, Japanese Patent Laid-Open No. 7-169040 and Japanese Patent Laid-Open No. 8-17037, polyurethane containing amine has been disclosed. In the foregoing disclosures, no description is made about the particle size of carbon black and combination of phenoxy resin and inorganic pigment.